The effect of nutrients such as glucose and protein and various hormones on hepatic insulin and C-peptide metabolism will be studied in dogs. The experiments will be conducted in conscious dogs three weeks after placement of catheters in the femoral artery, portal and hepatic veins. Constant infusion of indocyanine green and tritiated glucose will enable me to calculate hepatic plasma flow and glucose turnover respectively. Fluxes of insulin, C-peptide and glucose across the liver will be measured using levels obtained in the three vessels and hepatic insulin extraction assessed. In certain experiments renal insulin and C-peptide metabolism will be assessed together with hepatic extraction and the relationship of urinary C-peptide to portal insulin secretion will be evaluated. Hepatocytes isolated from these livers after perfusion with collagenase containing solutions, will be used to define the number and affinity of the insulin receptors on these cells. Hepatic insulin and C-peptide extraction will be calculated after an overnight fast and at successive time intervals following administration of different stimuli. Changes in hepatic glucose balance and glucose turnover will be followed. The number and affinity of the insulin receptors on hepatocytes will be measured at critical time points when maximal change in insulin extraction occurs in order to determine whether changes in insulin extraction are mediated at the receptor level. The hepatic extraction of C-peptide under different conditions will thus be compared to that of insulin. The use of the peripheral C-peptide/Insulin molar ratio represents a non-invasive approach to quantitate the hepatic extraction of insulin and its accuracy will be directly measured. In addition, exogenous infusion of C-peptide will allow me to measure simultaneously the hepatic, renal, metabolic and urinary clearance of C-peptide and document any change in these parameters under different physiological conditions. It is therefore anticipated that these studies will further our understanding of in vivo insulin and C-peptide metabolism and define the relationship between the metabolism of insulin and its biological activity.